Method for interference suppression of a sampling process and a device for carrying out the method

ABSTRACT

A method for interference suppression of a sampling process includes sampling an analog signal with a sampling frequency f, and determining whether an interference amplitude is present. The method provides that if an interference amplitude is present, the sampling frequency f is increased or decreased, and the method begins again with the sampling of the analog signal with the increased or decreased sampling frequency. In addition, a device is described for carrying out the method.

FIELD OF THE INVENTION

The present invention relates to a method for interference suppressionof a sampling process, in which, as a function of the presence of aparticular type of interference in the base band of a sampled andanalog-digital-converted signal, a successive modification of thesampling frequency is carried out.

BACKGROUND INFORMATION

For the digital evaluation of the measurement results of sensors, it isnecessary to convert the analog sensor signal, or the signal sampled bysensors, into a digital signal. Frequently, for this purposeanalog-digital converters are used, which make use of the advantages ofan oversampling. Here, the narrow-band input signal is sampled with ahigh-frequency clock rate, and is subsequently digitized using ananalog-digital converter. The bandwidth of the useful signal (alsoreferred to as useful band or base band) is here significantly smallerthan half the sampling frequency. If the input signal containshigh-frequency interference signals, these may be convoluted down intothe useful band due to the aliasing effect. In order to prevent this,standardly an anti-aliasing filter is used that filters out thehigh-frequency interference signals before the sampling.

FIG. 1 schematically shows a frequency diagram for such a situation.Analog useful signal 16 is sampled with a sampling frequency f. In anidealized circuit, or an idealized method, this takes place for exampleusing a periodic clock signal having the period T=1/f, a sampled valuebeing acquired in each case at a determined point in time within theperiod, for example when the periodic clock signal exceeds or fallsbelow a specified voltage, so that the temporal distance between twosuccessive sampling times corresponds in each case to the period T. Thesampled useful signal then results, in the time representation, as theproduct of the input signal times a sampling function that is given by asequence of equidistant sampling pulses with the temporal spacing T. Inthe frequency representation, this product corresponds to a convolutionof the frequency spectrum of the input signal with the frequencyspectrum of the sampling function, given by a sequence of equidistantspectral lines with the spacing f. Frequency spectrum 10 of a samplingfunction is shown in FIG. 1.

The input signal can contain superposed high-frequency interferencesignals, such as those that occur for example in the case ofelectromagnetic coupling in. According to the existing art, such aninterference signal is filtered out before the sampling using ananti-alias filter, for example a low-pass filter having transmissionfunction 14. Such a filtered high-frequency interference signal 12 isshown in FIG. 1 as an example, at a frequency that is slightly greaterthan twice the sampling frequency f. The point of intersection oftransmission function 14 of the anti-aliasing filter with the frequencyaxis limits base band 1 to a range that extends from 0*f up to afrequency of f/2. Here, the low-pass filter is fashioned such that ananalog useful signal 16 in base band 1 is allowed to pass through asunfiltered as possible at a typical useful frequency. A disadvantage ofthis solution is that an anti-aliasing filter has to be implemented.This results in development outlay and use of surface area, inparticular if an application-specific integrated circuit (ASIC) isrealized.

FIG. 2 shows the frequency spectra of the sampling of an analog usefulsignal 16 without anti-aliasing filter. Base band 1 extends, as in thecase of the filtering using an anti-aliasing filter, over a frequencyrange of from 0*f to f/2. However, interference amplitude 20, whichrepresents the convolution of an unfiltered high-frequency interferencesignal 18 in the case of sampling without the use of the anti-aliasingfilter, is for the most part convoluted directly into base band 1 andthus, given sampling with a sampling frequency f whose frequencyspectrum 10 in FIG. 2 is depicted as equal to that in FIG. 1, is wronglyinterpreted as useful signal 16.

In the article “Digital Alias-free signal processing in the GHzfrequency Range” by I. Bilinksis, G. Cain, published in 1996, pages6/1-6/6 (XP1133893), the authors discuss a sampling method freed fromaliasing, in which the sampling times are shifted in time by anarbitrary amount of time with respect to a periodic sampling.

Furthermore, U.S. Pat. No. 5,485,273 discusses a resolution systemreinforced by a ring laser gyroscope, in which a phase-locked loopcircuit used for sampling frequency modulation is used in combinationwith a fast filter.

Moreover, EP 1 330 036 A1 discusses a method and a device foralias-suppressed digitization of analog signals of a high frequency, inwhich a clock pulse generator generates a sequence of electrical pulsesof a predetermined frequency F_(clk), the sequence being divided by apseudo-arbitrary value for the purpose of selecting a pulse from thesequence.

SUMMARY OF THE INVENTION

According to the present invention, a method is provided forinterference suppression of a sampling process, the method including themethod steps of sampling an analog signal with a sampling frequency f,and of determining whether an interference amplitude is present. Themethod furthermore includes the step of an analog-digital conversion ofthe sampled signal, an interference amplitude being determined to bepresent, in the method step of determining whether an interferenceamplitude is present, only if it is greater than the noise of ananalog-digital conversion performed in connection with the method.

An interference amplitude is present only if the interference amplitudeis present in the base band of the frequency spectrum of a sampleduseful signal, the base band extending across a frequency range from 0 fto f/2. When an interference amplitude is present, sampling frequency fis increased or decreased, and the method begins anew with the methodstep of sampling the analog signal at the increased or decreasedsampling frequency. According to the present invention, the step ofincreasing or decreasing the sampling frequency f occurs successively bya predetermined constant absolute value Δf, in a direction having alwaysthe same sign, up to a threshold value f_(g). In other words, if aninterference signal is present, sampling takes place with a new,increased or decreased, sampling frequency. In the other case, samplingcontinues with the original sampling frequency f.

The advantage of such a method is that, given an analog-digitalconversion that does without an anti-aliasing filter, a high-frequencyinterference signal is no longer convoluted into the base band, butrather into a frequency range that is outside the base band. Thus, theanalog-digital-converted useful signal, due to the simple changing ofthe sampling frequency, can be correctly interpreted, and thus the useof an anti-aliasing filter can be omitted. This can save costs and spacein the implementation of an analog-digital converter. In addition, whenan anti-aliasing filter is used there is also an undesired attenuationof the useful signal, while not all frequency portions that continue tocause aliasing effects can be suppressed by the anti-aliasing filter.These disadvantages can also be avoided through the use of the methodaccording to the present invention.

In this way, the noise of an analog-digital conversion, applied in thecontext of the method, that is insignificant for the interpretation ofthe useful signal is excluded as a trigger for a modification of thesampling frequency. Through this, the method gains in efficiency,because a modification of the sampling frequency is carried out onlywhen the interpretation of the useful signal is impaired by ahigh-frequency interference signal or interference amplitude.

In this way, the method is initiated only when the interfering signalhas an effect in the useful band of the frequency range; this increasesthe efficiency of the method.

Through this deterministic modification of the sampling frequency, theeffect of the method on a high-frequency interference signal can bebetter determined, or more precisely regulated, because the modificationof the sampling frequency takes place in linear fashion, i.e. inequidistant steps. Through the limitation, via threshold value f_(g), ofthe possible modification of the sampling frequency, it can be ensuredthat the method is not carried out beyond a range in which it makessense to carry it out, thus preventing an addition of noise to theuseful signal in the base band.

In a development of this specific embodiment, Δf is formed according tothe following rule containing threshold value f_(G):f_(increased/decreased)=f+Δf, where Δf=(f_(g)−f_(1,0))/n, n∈Z, andf_(1,0) is the initial sampling frequency.

In a development of this specific embodiment, threshold value f_(G) isdefined by the value of the aperture jitter of a sample and hold circuitused in the sampling process. In this way, the maximum possiblethreshold value f_(g) is selected for the method, providing the widestrange of play for a modification of the sampling frequency.

In a specific embodiment, threshold value f_(G) corresponds to at mostthe initial sampling frequency increased or decreased by 5%.

In a specific embodiment, threshold value f_(G) corresponds to at mostthe initial sampling frequency increased or decreased by 1%.

In particular, n is selected from a subset of the whole numbers M, whereM∈[1, 20]. When parameter n is chosen in this way, the number ofpossible modification steps is limited to a number of steps that isadvantageous to realize in terms of circuitry. Still more particularly,M∈[2, 10].

In addition, a device is provided that includes a clock pulse generatorthat is configured to produce a periodic clock signal and to modify theperiod of the periodic clock signal upon reception of a control signal.In addition, the device includes a sampling unit that is configured touse the periodic clock signal for the sampling of an analog usefulsignal and to produce a sampled useful signal. In addition, ananalog-digital converter is configured to convert the sampled usefulsignal into a digital useful signal.

A unit for determining an interference amplitude is configured todetermine whether the digital useful signal is affected by aninterference amplitude. An interference amplitude is determined to bepresent only if it is greater than the noise of the performedanalog-digital conversion and if the interference amplitude is presentin the base band of the frequency spectrum of the sampled useful signal,the base band extending across a frequency range from 0*f to f/2. In theevent of the presence of an interference amplitude, the unit fordetermining an interference amplitude generates a control signal andsupplies it to the clock pulse generator in such a way that a change inthe sampling frequency f in the sampling unit occurs successively by apredetermined constant absolute value Δf, in a direction having alwaysthe same sign, up to a threshold value f_(g).

Exemplary embodiments of the present invention are explained in moredetail on the basis of the drawings and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the frequency spectra of the sampling of a useful signalusing an anti-aliasing filter known from the existing art.

FIG. 2 shows the frequency spectra of the sampling of a useful signalwithout the use of an anti-aliasing filter from the existing art.

FIG. 3 shows the frequency spectra of a specific sampling of a usefulsignal using the method according to the present invention.

FIG. 4 shows the design of a device for carrying out the methodaccording to the present invention.

FIG. 5 shows the design of a specific embodiment of a device forcarrying out the method according to the present invention.

DETAILED DESCRIPTION

In FIG. 3, the frequency spectra are shown of a specific sampling of auseful signal using the method according to the present invention. Baseband 1 extends over the frequency range from 0 f to f/2. An analoguseful signal 16 is sampled with a first sampling frequency 17. Ifinterference amplitude 20 of a high-frequency unfiltered interferingsignal 18 is present, this interference amplitude 20 is recognized inthe course of the method according to the present invention. Amodification 24 of sampling frequency 17 to an increased or decreasedsampling frequency 22 is then carried out in accordance with the method,and the method is restarted (not shown) with the increased or decreasedsampling frequency 22, with the method step of sampling analog signal16.

An interference amplitude 20 is recognized only if this amplitude isgreater than the noise 3 of an analog-digital conversion carried out inthe course of the method, and interference amplitude 20 additionallycomes to be situated in base band 1. Interference amplitude 20 isrecognized and, in the present exemplary embodiment, sampling frequency17 is decreased to a sampling frequency 22. In this way, there is ashift 25 of interference amplitude 20—previously convoluted into baseband 1—of high-frequency unfiltered interference signal 18 into afrequency range that is outside base band 1.

Here, the modification of sampling frequency 17 to sampling frequency 22takes place successively, by a predetermined constant magnitude, in adirection always having the same sign, until interfering amplitude 20 ofhigh-frequency unfiltered interference signal 18 is no longerrecognized, i.e. is no longer convoluted into base band 1, but ratherinto a frequency range outside base band 1. In this case, modification24 of sampling frequency 17 to decreased sampling frequency 22 is madeup of a multiplicity of, for example n, determined constant modificationsteps, always in a direction having the same sign, which in FIG. 3 isnegative. Here, the magnitude of modification 24 of sampling frequency17, i.e. the magnitude of its increase or, as in the present case,decrease, can be limited by a threshold value 15 beyond which no furthermodification of the sampling frequency is carried out.

Threshold value 15 can be based on the aperture jitter of a sample andhold circuit used in the method, but can also be limited to a value ofinitial sampling frequency 17 increased or decreased by at most X%,which may be 1% or 5%. The dimensioning of the magnitude by whichmodification 24 of sampling frequency 17 to sampling frequency 22 takesplace, successively and always in the same direction up to a maximum ofthreshold value 15, can for example also be carried out using theselected, or set, threshold value 15, via a formation rule in which thedifference of threshold value 15 and the first sampling frequency isdecomposed into n substeps.

FIG. 4 shows the design of a device for carrying out the methodaccording to the present invention. Realized here is a clock pulsegenerator 60 that is configured to generate a periodic clock signal 65and to modify the period of periodic clock signal 65 upon reception of acontrol signal 55. Periodic clock signal 65 is provided to a samplingunit 30 in which periodic clock signal 65 is used to sample an analoguseful signal 16. Sampling unit 30 is in addition configured to generatea sampled useful signal 38 and to provide it to an analog-digitalconverter 40.

Analog-digital converter 40 is configured to convert sampled usefulsignal 38 into a digital useful signal 45 and to supply it to a unit 50for determining interference amplitudes. This unit 50 for determininginterference amplitudes determines whether digital useful signal 45 isaffected by an interference amplitude 20, or whether an interferenceamplitude 20 is present. If an interference amplitude 20 is present, acontrol signal 55 is generated in unit 50 for determining interferenceamplitudes and is supplied to clock pulse generator 60, which thereuponmodifies the period of periodic clock signal 65. If no interferenceamplitude 20 is present, no control signal 55 is generated. Samplingunit 30, analog-digital converter 40, unit 50 for determininginterference amplitudes, and clock pulse generator 60 thus form aclosed-loop control circuit.

In the device depicted here, clock pulse generator 60, sampling unit 30,analog-digital converter 40, and unit 50 for determining interferenceamplitudes are shown as separate units or components. These componentscan however be combined with one another in any manner, both spatiallyand functionally. Thus, purely as an example, both the sampling and theanalog-digital conversion can take place in one element.

FIG. 5 shows a specific embodiment of a device for carrying out themethod according to the present invention. Shown are an analog-digitalconverter 40 and a sampling unit 30, shown as a functional unit foranalog-digital conversion 30, 40. Sampling unit 30 is realized here as asample and hold circuit whose switching equipment and hold capacitor areshown schematically in FIG. 5. The functional unit for analog-digitalconversion 30, 40 is connected to a unit 50 for determining interferenceamplitudes, which is realized in this exemplary embodiment as a digitalpart having a digital comparator. The digital part is in turn connectedto a clock pulse generator 60, realized as an oscillator, while clockpulse generator 60 is connected to the functional unit foranalog-digital conversion 30, 40.

Thus, a closed-loop control circuit is present via the functional unitfor analog-digital conversion 30, 40, clock pulse generator 60, and unit50 for determining interference amplitudes. In the oscillator, aperiodic clock signal 65 is generated having period T and frequencyf=1/T. This periodic clock signal 65 is provided to the sample and holdcircuit and is used to open and close the switching equipment of thesample and hold circuit with frequency f. If in addition to periodicclock signal 65 an analog useful signal 16 is provided to the sample andhold circuit, this analog useful signal 16 is sampled with a firstsampling frequency 17 that corresponds to the above-indicated frequencyf, and is converted from analog to digital, likewise in the functionalunit for analog-digital conversion 30, 40. At the output of thefunctional unit for analog-digital conversion 30, 40 there is present adigital useful signal 45 that is supplied to the digital part.

In the digital comparator of the digital part, digital useful signal 45is compared to at least one reference value that corresponds for exampleto the value of the average amplitude of noise 3 of the upstreamanalog-digital conversion. If digital useful signal 45 is greater thanthe at least one reference value, then in the digital part a controlsignal 55, which may be for example a 10-bit trimming signal, isgenerated and is supplied to the oscillator. In this oscillator, theperiod of periodic clock signal 65 is then modified corresponding tocontrol signal 55, to an increased or decreased sampling frequency 22.From the clock pulse curve shown schematically in FIG. 5 (at thefunctional block of the oscillator), it can be seen that samplingfrequency 22 is greater than sampling frequency 17. The samplingfrequency is adapted until digital useful signal 45 is smaller than theat least one reference value of the digital comparator of digital part50, or until a specified threshold value 15 of the maximum frequencymodification has been reached.

In all exemplary embodiments, upon the reaching of the threshold value15, for example an initial sampling frequency 17 increased by X%, themethod can be restarted with an initial sampling frequency 17 decreasedby X%, so that, despite the presence of an interference amplitude 20 inbasis been 1, the sampling frequency is continuously modified in a loop.Upon reaching a threshold value 15 of, for example, an initial samplingfrequency 17 decreased by X%, the method correspondingly begins at aninitial sampling frequency 17 increased by X%.

1-10. (canceled)
 11. A method for providing interference suppression ofa sampling process, the method comprising: sampling an analog usefulsignal with a sampling frequency f; providing analog-digital conversionof the sampled useful signal; determining whether an interferenceamplitude is present, an interferences amplitude being determined to bepresent only if it is greater than the noise of the analog-digitalconversion performed in connection with the method and if theinterference amplitude is present in the base band of the frequencyspectrum of the sampled useful signal, the base band extending across afrequency range from 0*f to f/2; increasing or decreasing the samplingfrequency f if an interference amplitude is present, then beginning anewwith the sampling of the analog useful signal with the increased ordecreased sampling frequency; wherein the increasing or decreasing ofthe sampling frequency f is performed successively by a predetermined,constant absolute value Δf in a direction having always the same sign,up to a threshold value f_(g).
 12. The method of claim 11, wherein Δf isformed according to the following rule containing threshold value f_(G):f_(increased/decreased)=f+Δf, where Δf=(f_(g)−f_(1,0))/n, n∈Z, andf_(1,0) is the initial sampling frequency.
 13. The method of claim 11,wherein the threshold value f_(G) is defined through the value of theaperture jitter of a sample and hold circuit used in the context of thesampling process.
 14. The method of claim 11, wherein the thresholdvalue f_(G) corresponds at most to the initial sampling frequencyincreased or decreased by 5%.
 15. The method of claim 11, wherein thethreshold value f_(G) corresponds at most to the initial samplingfrequency increased or decreased by 1%.
 16. The method of claim 12,wherein n is selected from a subset of the whole numbers M, where M∈[1,20].
 17. A device, comprising: a clock pulse generator to produce aperiodic clock signal and to modify a period of the periodic clocksignal upon reception of a control signal; a sampling unit to use theperiodic clock signal as a sampling frequency f for the sampling of ananalog useful signal and to produce a sampled useful signal; ananalog-digital converter to convert the sampled useful signal into adigitally useful signal; an interference amplitude determination unit todetermine whether the digitally useful signal is affected by aninterference amplitude, an interference amplitude being determined to bepresent only if it is greater than the noise of the performedanalog-digital conversion and if the interference amplitude is presentin the base band of the frequency spectrum of the sampled useful signal,the base band extending across a frequency range from 0*f to f/2;wherein if an interference amplitude is present, the interferenceamplitude determination unit generates a control signal and supplies itto the clock pulse generator so that a change in the sampling frequencyfin the sampling unit occurs successively, by a predetermined constantabsolute value Δf in a direction having always the same sign, up to athreshold value f_(g).
 18. The device of claim 17, wherein Δf is formedaccording to the following rule containing threshold value f_(G):f_(increased/decreased)=f+Δf, where Δf=(f_(g)−f_(1,0))/n, n∈Z, andf_(1,0) is the initial sampling frequency.
 19. The device of claim 17,wherein the threshold value f_(G) is defined through the value of theaperture jitter of a sample and hold circuit used in the context of thesampling process.
 20. The device of claim 17, wherein the thresholdvalue f_(G) corresponds at most to the initial sampling frequencyincreased or decreased by 5%.
 21. The device of claim 17, wherein thethreshold value f_(G) corresponds at most to the initial samplingfrequency increased or decreased by 1%.
 22. The device of claim 18,wherein n is selected from a subset of the whole numbers M, where M∈[1,20].